Solid electrolyte and all solid state battery using the same

ABSTRACT

A solid electrolyte of the present invention is represented by a general formula: Li a P b M c O d N e , where M is at least one element selected from the group consisting of Si, B, Ge, Al, C, Ga and S, and a, b, c, d and e respectively satisfy a=0.62 to 4.98, b=0.01 to 0.99, c=0.01 to 0.99, d=1.070 to 3.985, e=0.01 to 0.50, and b+c=1.0. The solid electrolyte hardly deteriorates in a wet atmosphere.

RELATED APPLICATION

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/JP2004/009302, filed on Jun. 24, 2004,which in turn claims the benefit of Japanese Application No.2003-184625, filed on Jun. 27, 2003, the disclosure of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to an all solid state battery,particularly to a solid electrolyte used in an all solid state thin filmlithium secondary battery.

BACKGROUND ART

With the development in portable devices such as personal computers andmobile phones, demand is growing for batteries as power sources for suchdevices.

In batteries for such application, an electrolyte composed of a liquidsuch as organic solvent has been used as a medium for transferring ions.For this reason, there is a possibility that problems such as leakage ofelectrolyte from the battery might occur.

In order to solve the above problems, development is under way toproduce an all solid state battery using, instead of a liquidelectrolyte, a solid electrolyte. An all solid state lithium secondarybattery, in particular, is vigorously being studied in many fields as abattery capable of providing a high energy density. This is because Lihas a low atomic weight, the greatest ionization tendency, and thelowest reduction potential, and thus, for example, the use of Li metalin a negative electrode active material yields a high electromotiveforce.

Well-known examples of the solid electrolyte used for the all solidstate lithium secondary battery are lithium halide, lithium nitride,lithium oxyacid salts and derivatives thereof. For example, U.S. Pat.No. 5,597,660 reports in the specification that lithium phosphorusoxynitride (Li_(x)PO_(y)N_(z), where x, y and z satisfy x=2.8 and3z+2y=7.8) obtained by introducing nitrogen (N) into lithiumorthophosphate (Li₃PO₄) has a very high lithium ion conductivity of1×10⁻⁶ to 2×10⁻⁶ S/cm although it is an oxide-based material.

When the lithium phosphorus oxynitride is exposed to a wet atmosphere,however, phosphorus atoms (P) forming the lithium phosphorus oxynitridereact with water molecules present in the wet atmosphere, during whichthe phosphorus atoms are reduced to a lower oxidation state from anoxidation state of +5. Thereby, lithium phosphorus oxynitride isdecomposed, which significantly decreases the ion conductivity thereof.

When such decrease in ion conductivity occurs in an all solid statebattery using a solid electrolyte composed of lithium phosphorusoxynitride, internal impedance will increase. For this reason, itscharge/discharge rate characteristics will be significantly impaired.

In view of the above, an object of the present invention is to provide asolid electrolyte capable of preventing the ion conductivity fromdecreasing even in a wet atmosphere, and an all solid state batteryusing the solid electrolyte.

DISCLOSURE OF INVENTION

The solid electrolyte of the present invention is represented by ageneral formula:

Li_(a)P_(b)M_(c)O_(d)N_(e), where M is at least one element selectedfrom the group consisting of Si, B, Ge, Al, C, Ga and S, and where a, b,c, d and e respectively satisfy a=0.62 to 4.98, b=0.01 to 0.99, c=0.01to 0.99, d=1.070 to 3.985, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=0.62 to 2.98, b=0.01 to 0.99,c=0.01 to 0.99, d=1.070 to 3.965, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=1.61 to 2.99, b=0.01 to 0.99,c=0.01 to 0.99, d=2.060 to 3.975, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=1.61 to 2.99, b=0.01 to 0.99,c=0.01 to 0.99, d=3.050 to 3.985, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=2.6 to 3.0, b=0.01 to 0.99,c=0.01 to 0.99, d=2.60 to 3.975, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=2.61 to 3.99, b=0.01 to 0.99,c=0.01 to 0.99, d=3.050 to 3.985, e=0.01 to 0.50, and b+c=1.0.

In the formula, it is preferred that a=2.62 to 4.98, b=0.01 to 0.99,c=0.01 to 0.99, d=3.050 to 3.985, e=0.01 to 0.50, and b+c=1.0.

The present invention further relates to an all solid state batterycomprising a positive electrode, a negative electrode and the aforesaidsolid electrolyte disposed between the positive electrode and thenegative electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view of a test cell used for theevaluation of solid electrolytes in Examples of the present invention.

FIG. 2 is a schematic cross sectional view of an all solid state batteryin Examples of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The solid electrolyte of the present invention is composed of Li(lithium), O (oxygen), N (nitrogen), P (phosphorous) and at least oneelement M selected from the group consisting of Si (silicon), B (boron),Ge (germanium), Al (aluminum), C (carbon), Ga (gallium) and S (sulfur).

For example, the solid electrolyte can be composed of a nitride of alithium oxyacid salt containing phosphorus and the element M. In thiscase, phosphorus and the element M may be mixed at atomic level to forma nitride of the lithium oxyacid salt. Alternatively, lithium phosphorusoxynitride, which is a nitride of a lithium oxyacid salt containingphosphorus, and a nitride of a lithium oxyacid salt containing theelement M may be mixed at particle level.

The electrolyte of the present invention is represented by the generalformula Li_(a)P_(b)M_(c)O_(d)N_(e), where M is at least one elementselected from the group consisting of Si, B, Ge, Al, C, Ga and S, andwhere a, b, c, d and e respectively satisfy a=0.62 to 4.98, b=0.01 to0.99, c=0.01 to 0.99, d=1.070 to 3.985, e=0.01 to 0.50, and b+c=1.0.

When lithium phosphorus oxynitride, which is a conventionally used solidelectrolyte, is allowed to stand in a wet atmosphere, it will readilyreact with water, which significantly decreases the ion conductivity.This is because some proportion of P (phosphorus) contained in thelithium phosphorus oxynitride reacts with water present in theatmosphere, and is reduced from a valence of +5.

The solid electrolyte of the present invention, on the other hand,contains the element M capable of thermodynamically forming a stablerbond with oxygen than a bonding state between phosphorus and oxygen inthe lithium phosphorus oxynitride. This stabilizes the structure of thesolid electrolyte and improves the reduction resistance of phosphorus,allowing the P in the solid electrolyte to retain a valence of +5. Forthis reason, it is possible to prevent the ion conductivity of the solidelectrolyte from decreasing in a wet atmosphere.

In the above general formula, when c is 0.01 to 0.99, the decrease inion conductivity due to storage in a wet atmosphere can be prevented.When c is less than 0.01, the reduction of phosphorus cannot beprevented sufficiently. Preferably, c is 0.10 to 0.99. When a nitride ofa lithium oxyacid salt containing phosphorus and the element M is usedto obtain a solid electrolyte, the nitride can form a solid solution,thus yielding a solid electrolyte which is chemically stable in aresultant battery.

Further, it is particularly preferred that c be 0.1 to 0.5. Byincreasing the concentration of phosphorus in the solid electrolyte ofthe present invention, it is possible to obtain a solid electrolytewhich is not only chemically stable even if it contacted Li metal, butalso has higher lithium ion conductivity.

When e is 0.01 to 0.50, high ion conductivity will be obtained, and thedecrease in ion conductivity due to storage in a wet atmosphere will beprevented. When e is less than 0.01, high ion conductivity will hardlybe retained. Further, if a nitride of a lithium oxyacid salt is used toobtain a solid electrolyte, when e exceeds 0.50, the framework structureof the lithium oxyacid salt will be damaged, which is likely to resultin a decrease in ion conductivity. The use of such solid electrolytehaving decreased ion conductivity in a thin film all solid statesecondary battery increases the resistance of the solid electrolyte,which significantly impairs charge/discharge characteristics.

The composition of the electrolyte varies according to the type of theelement M used, the ratio of the element M to phosphorus in the solidelectrolyte of the present invention, etc. In other words, a, b and d inthe above general formula depend on the composition or type of thelithium oxyacid salt containing the element M used as the raw material,and the ratio of the element M to phosphorus in the solid electrolyte ofthe present invention. Accordingly, a is in a range of 0.62 to 4.98. bis in a range of 0.01 to 0.99. d is in a range of 1.070 to 3.985.

The above-described solid electrolyte may further contain an elementother than those listed above as long as the effect of the presentinvention is not impaired.

The solid electrolyte of the present invention can also be made from,for example, lithium orthophosphate (Li₃PO₄), which is a lithium oxyacidsalt containing phosphorus, and a lithium oxyacid salt containing theelement M by partially substituting nitrogen for oxygen. The lithiumoxyacid salt containing the element M as the raw material may be asingle compound or a mixture of two or more. Further, other than lithiumorthophosphate, other lithium phosphate (e.g., LiPO₃) or a mixture ofLi₂O and P₂O₅ can also be used as the lithium oxyacid salt containingphosphorus. Further, other than the lithium oxyacid salt containing theelement M, a mixture of Li₂O and a lithium oxyacid salt containing theelement M, or a mixture of Li₂O and an oxide containing the element Mmay be used. Further, the solid electrolyte of the present invention maybe made from lithium phosphorus oxynitride and a nitride of a lithiumoxyacid salt containing the element M.

For example, when lithium orthophosphate and any of LiBO₂, LiAlO₂ andLiGaO₂ are used as the raw material, in other words, when producing asolid oxide represented by the previously described general formula,where M is B, Al or Ga, it is preferred that a=0.62 to 2.98, b=0.01 to0.99, c=0.01 to 0.99, d=1.070 to 3.965, e=0.01 to 0.50, and b+c=1.

For example, when lithium orthophosphate and any of Li₂SiO₃, Li₂GeO₃ andLi₂CO₃ are used as the raw material, in other words, when producing asolid oxide represented by the previously described general formula,where M is Si, Ge or C, it is preferred that a=1.61 to 2.99, b=0.01 to0.99, c=0.01 to 0.99, d=2.060 to 3.975, e=0.01 to 0.50, and b+c=1.

For example, when lithium orthophosphate and Li₂SO₄ are used as the rawmaterial, in other words, when producing a solid oxide represented bythe previously described general formula, where M is S, it is preferredthat a=1.61 to 2.99, b=0.01 to 0.99, c=0.01 to 0.99, d=3.050 to 3.985,e=0.01 to 0.50, and b+c=1.

For example, when lithium orthophosphate and Li₃BO₃ are used as the rawmaterial, in other words, when producing a solid oxide represented bythe previously described general formula, where M is B, it is preferredthat a=2.6 to 3.0, b=0.01 to 0.99, c=0.01 to 0.99, d=2.060 to 3.975,e=0.01 to 0.50, and b+c=1.

For example, when lithium orthophosphate and either Li₄SiO₄ or Li₄GeO₄are used as the raw material, in other words, when producing a solidoxide represented by the previously described general formula, where Mis Si or Ge, it is preferred that a=2.61 to 3.99, b=0.01 to 0.99, c=0.01to 0.99, d=3.050 to 3.985, e=0.01 to 0.50, and b+c=1.

For example, when lithium orthophosphate and Li₅AlO₄ are used as the rawmaterial, in other words, when producing a solid oxide represented bythe previously described general formula, where M is Al, it is preferredthat a=2.62 to 4.98, b=0.01 to 0.99, c=0.01 to 0.99, d=3.050 to 3.985,e=0.01 to 0.50, and b+c=1.

Lithium orthophosphate and the above-listed lithium oxyacid salts shouldbe used to satisfy the previously described general formula.

The solid electrolyte of the present invention is preferably a thinfilm. The thickness thereof can be appropriately adjusted, and thepreferred thickness is 0.1 to 10 μm.

The solid electrolyte of the present invention may be either crystallineor amorphous.

Further, as the solid electrolyte of the present invention, phosphorusand the element M may be mixed at atomic level to form a nitride of alithium oxyacid salt in solid solution. Alternatively, lithiumphosphorus oxynitride, which is a nitride of a lithium oxyacid saltcontaining phosphorus, and a nitride of a lithium oxyacid saltcontaining the element M may be mixed at particle level.

As for the method for producing a solid electrolyte of the presentinvention, similar to the method for producing a simple substance oflithium phosphorus oxynitride, which is a conventional solidelectrolyte, there can be used, for example, a thin film formingtechnique using a vacuum apparatus. It is needless to say that a methodother than this can also be used.

Examples of the method for forming a thin film composed of a solid oxideof the present invention include sputtering method in which a target issputtered using nitrogen (N₂) by means of a magnetron or high frequencyand a combined method of vapor deposition method and ion beamirradiation for introducing nitrogen ions. Examples of the vapordeposition include resistance heating vapor deposition method in whichvapor deposition is performed by heating a vapor deposition source usinga resistance; electron beam vapor deposition method in which vapordeposition is performed by heating a vapor deposition source using anelectron beam; and laser ablation method in which vapor deposition isperformed by heating a vapor deposition source using a laser.

In the vapor deposition, as the target or vapor deposition source, theuse of lithium orthophosphate (Li₃PO₄) and a lithium oxyacid saltcontaining the element M is necessary.

For example, in the case of sputtering method, as the target, lithiumorthophosphate as a lithium oxyacid salt containing phosphorus and alithium oxyacid salt containing the element M are used. For example, inthe case of resistance heating vapor deposition method, electron beamvapor deposition method and laser ablation method, as the vapordeposition source, lithium orthophosphate as a lithium oxyacid saltcontaining phosphorus and a lithium oxyacid salt containing the elementM are used.

In either of the sputtering method and vapor deposition method, oxygencontained in the lithium orthophosphate and oxygen contained in thelithium oxyacid salt containing the element M can be partially nitrifiedsimultaneously by introducing nitrogen.

Further, resistance heating vapor deposition method using lithiumorthophosphate as the vapor deposition source and electron beam vapordeposition method using a lithium oxyacid salt containing the element Mas the vapor deposition source can be combined. Other than this, thecombination of resistance heating vapor deposition method and laserablation method, and the combination of electron beam vapor depositionmethod and laser ablation method are also possible.

A mixture of lithium oxyacid salt obtained by mixing a lithium oxyacidsalt containing the element M with lithium phosphate at a given mixingratio may be used as the target or vapor deposition source.

As the target or vapor deposition source, other than the above-mentionedlithium orthophosphate, other lithium phosphate (e.g., LiPO₃) and amixture of Li₂O and P₂O₅, which are lithium oxyacid salts containingphosphorus, can also be used. Further, other than a lithium oxyacid saltcontaining the element M, a mixture of Li₂O and a lithium oxyacid saltcontaining the element M, or a mixture of Li₂O and any of SiO₂, Bi₂O₃,GeO₂, Al₂O₃ and Ga₂O₃ may be used.

An all solid state battery of the present invention can be obtained byusing the above-described solid electrolyte.

As an example of an all solid state battery using the solid electrolyteof the present invention, FIG. 2 shows a schematic cross sectional viewof an all solid state thin film lithium secondary battery.

The all solid state thin film lithium secondary battery comprises a baseplate 21, and a first current collector 22, a first electrode 23, asolid electrolyte 24 of the present invention, a second electrode 25 anda second current collector 26 which are formed on the base plate 21. Inthis case, the first electrode serves as the positive electrode layer,and the second electrode serves as the negative electrode layer.However, the first electrode may serve as the negative electrode layer,and the second electrode may serve as the positive electrode layer.

This battery can be obtained by laminating, on the base plate 21, thefirst current collector 22, the first electrode 23, the solidelectrolyte 24, the second electrode 25, and the second currentcollector 26 in this order using a thin film forming method using avacuum apparatus. It is needless to say that a method other than thethin film forming method using a vacuum apparatus can be used. Further,a resin or aluminum laminate film may be placed on the second currentcollector 26 to form a protection layer.

As the base plate 21, there can be used an electrically insulating baseplate such as alumina, glass or polyimide film; a semiconductor baseplate such as silicon; or a electron conductive base plate such asaluminum or copper. In the case of using the electron conductive baseplate, in order to prevent the first current collector 22 fromelectrically connecting to the second current collector 26, anelectrically insulating material is placed on at least either of theinterface between the first current collector 22 and the base plate 21or the interface between the second current collector 26 and the baseplate 21. Because the base plate preferably has a low surface roughness,it is effective to use a plate having mirror-finished surface or thelike.

As the first current collector 22 placed on the base plate 21, forexample, there can be used an electron conductive material such asplatinum, platinum/palladium, gold, silver, aluminum, copper or ITO(indium-tin oxide film). Other than those listed above, any materialhaving electron conductivity and unreactive with the first electrode 23can be used as the current collector.

As for the method for producing the first current collector 22, therecan be used sputtering method, resistance heating vapor depositionmethod, ion beam vapor deposition method or electron beam vapordeposition method. When the base plate 21 is composed of an electronconductive material such as aluminum, copper or stainless steel, thefirst current collector 22 may be omitted.

The first electrode (positive electrode layer) 23 is preferably composedof, for example, a positive electrode material for lithium secondarybatteries such as lithium cobaltate (LiCoO₂), lithium nickelate (LiNiO₂)or lithium manganate (LiMn₂O₄), and a transition metal oxide such asvanadium oxide (V₂O₅), molybdenum oxide (MoO₃) or titanium sulfide(TiS₂). Besides those listed above, any material that can be used for apositive electrode for lithium secondary batteries can be used for thefirst electrode 23.

As for the method for producing the first electrode (positive electrodelayer) 23, there can be used sputtering method, resistance heating vapordeposition method, ion beam vapor deposition method, electron beam vapordeposition method or laser ablation method.

As the solid electrolyte 24, the solid electrolyte of the presentinvention described previously is used.

The second electrode (negative electrode layer) 25 is preferablycomposed of, for example, a negative electrode material for lithiumsecondary batteries such as a carbon material (C) including graphite orhard carbon, any of an alloy containing tin (Sn), lithium cobalt nitride(LiCoN), lithium metal (Li) or a lithium alloy (e.g. LiAl). Other thanthose listed above, any material that can be used as a negativeelectrode for lithium secondary batteries can be used for the secondelectrode 25.

As for the method for forming the second electrode (negative electrodelayer) 25, there can be used sputtering method, resistance heating vapordeposition method, ion beam vapor deposition method, electron beam vapordeposition method or laser ablation method.

As the second current collector 26, the materials listed in the case ofthe first current collector 22 can be used. Similarly, as the method forproducing the second current collector 26, the methods listed in thecase of the first current collector 22 can be used.

It is also possible to stack a plurality of such all solid statebatteries to form a stacked battery.

Although this embodiment has been described for the case of using an allsolid state thin film lithium secondary battery as an example of the allsolid state battery of the present invention, it should be understoodthat the present invention is not limited thereto.

Hereinafter, the present invention will be described using examples, butit should be understood that the present invention is not limitedthereto.

Examples 1 to 10

Test cells were produced by the following procedure so as to evaluatesolid electrolytes.

In the first step, at a predetermined position on a silicon base plate11 having a mirror-finished surface having been oxidized and a surfaceroughness of not greater than 30 nm was placed a metal mask having anopening with a size of 20 mm×10 mm, after which rf magnetron sputteringmethod was performed to form a film composed of platinum. Thereby, aplatinum current collector layer 12 having a thickness of 0.5 μm wasobtained.

Subsequently, in the second step, on the thus-obtained platinum currentcollector layer 12 was placed a metal mask having an opening with a sizeof 15 mm×15 mm, after which rf magnetron sputtering method was performedto form a solid electrolyte layer 13 having a thickness of 1.0 μm andthe composition shown in Table 2.

In the rf magnetron sputtering method, lithium orthophosphate (Li₃PO₄)and a lithium oxyacid salt shown in Table 1 were used as the target. Thesputtering gas used here was nitrogen (N₂).

TABLE 1 Lithium oxyacid salt used as target Example 1 Li₄SiO₄ Example 2Li₂SiO₃ Example 3 LiBO₂ Example 4 Li₂GeO₃ Example 5 Li₄GeO₄ Example 6LiAlO₂ Example 7 Li₅AlO₄ Example 8 Li₂CO₃ Example 9 LiGaO₂ Example 10Li₂SO₄

The conditions for the rf magnetron sputtering method were as follows.The chamber internal pressure was 2.7 Pa. The amount of gas introducedwas 10 sccm. The high frequency power irradiated to the lithiumorthophosphate target was 200 W. The sputtering time was two hours.Further, the high frequency power irradiated to the element M-containinglithium oxyacid salt target was controlled so as to obtain a solidelectrolyte of lithium oxyacid salt including phosphorus and element Mat a molar ratio of 1:4 and having the composition shown in Table 2.

In the third step, on the solid electrolyte layer 13 produced above wasplaced a metal mask having an opening with a size of 10 mm×10 mm suchthat the mask did not extend beyond the solid electrolyte layer 13,after which rf magnetron sputtering method was performed to form a filmcomposed of platinum. Thereby, a platinum current collector layer 14having a thickness of 0.5 μm was obtained.

Comparative Example 1

In the second step, with the use of lithium orthophosphate as thetarget, a solid electrolyte thin film composed of lithium phosphorusoxynitride (Li_(2.8)PO_(3.45)N_(0.3)) was formed in the same manner asin Example 1. Thereby, a solid electrolyte having a thickness of 1.0 μmwas obtained. A test cell was produced in the same manner as in Example1 except for this second step.

[Evaluation]

In order to evaluate the solid electrolyte films in terms of waterresistance, the test cells of Examples 1 to 10 and Comparative Example 1produced above were stored in a controlled chamber with a humidity of50% and a temperature of 20° C. for two weeks. For each of the testcells, alternating current impedance was measured five times:immediately after the production, one day after the storage, two daysafter the storage, one week after the storage and two weeks after thestorage, so as to check a change in ion conductivity with time. Theconditions for the alternating current impedance measurement were asfollows. The equilibrium voltage was zero, the amplitude of the appliedvoltage was ±10 mV, and the range of the frequency used was 10⁵ to 0.1Hz. Ion conductivity was determined from the result of the alternatingcurrent impedance measurement.

The evaluation results are shown in Table 2. Note that the ionconductivity is expressed in relative value with the ion conductivityobtained from the result of the impedance measurement done immediatelyafter the production of the test cells set at 100.

TABLE 2 Ion conductivity Immediately 2 days 1 week 2 weeks after 1 dayafter after after after Solid electrolyte layer production storagestorage storage storage Ex. 1 Li_(3.0)P_(0.8)Si_(0.2)O_(3.45)N_(0.3)100.00 96.36 94.09 90.91 90.00 Ex. 2Li_(2.6)P_(0.8)Si_(0.2)O_(3.25)N_(0.3) 100.00 93.53 89.41 87.65 87.06Ex. 3 Li_(2.4)P_(0.8)B_(0.2)O_(3.05)N_(0.3) 100.00 90.90 84.30 80.9980.17 Ex. 4 Li_(2.6)P_(0.8)Ge_(0.2)O_(3.25)N_(0.3) 100.00 85.33 78.6774.67 74.67 Ex. 5 Li_(3.8)P_(0.8)Ge_(0.2)O_(3.45)N_(0.3) 100.00 94.8192.45 90.57 90.09 Ex. 6 Li_(2.4)P_(0.8)Al_(0.2)O_(3.05)N_(0.3) 100.0090.98 86.89 83.61 82.79 Ex. 7 Li_(3.2)P_(0.8)Al_(0.2)O_(3.45)N_(0.3)100.00 94.53 91.05 88.06 87.56 Ex. 8Li_(2.6)P_(0.8)C_(0.2)O_(3.25)N_(0.3) 100.00 88.28 82.07 77.93 77.24 Ex.9 Li_(2.4)P_(0.8)Ga_(0.2)O_(3.05)N_(0.3) 100.00 92.50 86.67 83.33 82.50Ex. 10 Li_(2.6)P_(0.8)S_(0.2)O_(3.45)N_(0.3) 100.00 84.44 80.74 77.0476.30 Comp. Li_(2.8)PO_(3.45)N_(0.3) 100.00 71.43 50.00 35.71 14.29 Ex.1

Table 2 indicates that, for the solid electrolytes of Examples 1 to 10,the decrease of ion conductivity was prevented even after storage in thewet atmosphere. However, for the solid electrolyte of ComparativeExample 1 without the element M, the ion conductivity decreasedsignificantly after storage.

The foregoing illustrates that the deterioration of the solidelectrolytes was prevented in Examples 1 to 10.

Examples 11 to 18 and Comparative Example 2

In the second step, lithium orthosilicate (Li₄SiO₄) was used as alithium oxyacid salt. The high frequency power irradiated to the lithiumorthosilicate target was controlled so as to obtain solid electrolyteshaving the compositions shown in Table 3. In other words, solidelectrolytes represented by a general formulaLi_(a)P_(b)Si_(c)O_(d)N_(e), where c was varied in the range of 0.005 to0.99 were produced. Test cells were produced in the same manner as inExample 1 except for the second step.

Subsequently, for each of the test cells, evaluation was madeimmediately after the production and two weeks after the storage in thesame manner as in Example 1. The evaluation results are shown in Table3. Note that the ion conductivity is expressed in relative value withthe ion conductivity obtained immediately after the production of thetest cell set at 100. Further, the ion conductivity immediately afterthe production is expressed in relative value with the ion conductivityof Comparative Example 2 set at 100.

TABLE 3 Ion con- ductivity Ion conductivity relative Immediately 2 weeksto that after after of Comp. Solid electrolyte layer production storageEx. 2 Comp. Li_(2.805)P_(0.995)Si_(0.005)O_(3.45)N_(0.3) 100.00 31.23100.00 Ex. 2 Ex. 11 Li_(2.81)P_(0.99)Si_(0.01)O_(3.45)N_(0.3) 100.0073.64 100.00 Ex. 12 Li_(2.85)P_(0.95)Si_(0.05)O_(3.45)N_(0.3) 100.0083.18 97.73 Ex. 13 Li_(2.9)P_(0.9)Si_(0.1)O_(3.45)N_(0.3) 100.00 88.1895.45 Ex. 14 Li_(3.0)P_(0.8)Si_(0.2)O_(3.45)N_(0.3) 100.00 90.00 86.36Ex. 15 Li_(3.3)P_(0.5)Si_(0.5)O_(3.45)N_(0.3) 100.00 90.45 84.09 Ex. 16Li_(3.4)P_(0.4)Si_(0.6)O_(3.45)N_(0.3) 100.00 89.91 59.09 Ex. 17Li_(3.7)P_(0.1)Si_(0.9)O_(3.45)N_(0.3) 100.00 89.09 34.09 Ex. 18Li_(3.79)P_(0.01)Si_(0.99)O_(3.45)N_(0.3) 100.00 87.27 32.73

As is evident from Table 3, for the solid electrolytes of Examples 11 to18 which were represented by a general formulaLi_(a)P_(b)Si_(c)O_(d)N_(e) where c was 0.01 or greater, the decrease inion conductivity was prevented after storage in the wet atmosphere.Particularly, for those of Examples 13 to 18 when c was 0.1 to 0.99, thedecrease in ion conductivity was further prevented. For the solidelectrolyte of Comparative Example 2 where c was 0.005, however, the ionconductivity decreased significantly after the storage.

Table 3 also indicates that high ion conductivity was obtained inExamples 11 to 15 where c was 0.5 or less.

This illustrates that the solid electrolytes of the present inventionproduced by using lithium phosphate and lithium orthosilicate (Li₄SiO₄)as a raw material can be represented by a general formulaLi_(a)P_(b)Si_(c)O_(d)N_(e), and that, when c is 0.01 to 0.99, thedecrease in ion conductivity due to storage in a wet atmosphere isprevented. It has been found that, particularly, c is preferably 0.1 to0.99, and more preferably, 0.1 to 0.5.

Examples 19 to 24 and Comparative Example 3

In the second step, lithium germanate (Li₄GeO₄) was used as a lithiumoxyacid salt. The high frequency power irradiated to the lithiumgermanate target was controlled so as to obtain solid electrolyteshaving the compositions shown in Table 4. In other words, solidelectrolytes represented by a general formulaLi_(a)P_(b)Ge_(c)O_(d)N_(e), where c was varied in the range of 0.005 to0.99 were produced. Test cells were produced in the same manner as inExample 1 except for the second step.

Subsequently, for each of the test cells, evaluation was madeimmediately after the production and two weeks after the storage in thesame manner as in Example 1. The evaluation results are shown in Table4. Note that the ion conductivity is expressed in relative value withthe ion conductivity obtained immediately after the production of thetest cell set at 100. Further, the ion conductivity immediately afterthe production is expressed in relative value with the ion conductivityof Comparative Example 3 set at 100.

TABLE 4 Ion con- Ion conductivity ductivity Im- relative mediately 2weeks to that after after of Comp. Solid electrolyte layer productionstorage Ex. 3 Comp. Li_(2.805)P_(0.995)Ge_(0.005)O_(3.45)N_(0.3) 100.0032.41 100.00 Ex. 3 Ex. 19 Li_(2.81)P_(0.99)Ge_(0.01)O_(3.45)N_(0.3)100.00 76.42 100.00 Ex. 20 Li_(2.9)P_(0.9)Ge_(0.1)O_(3.45)N_(0.3) 100.0087.26 97.67 Ex. 21 Li_(3.0)P_(0.8)Ge_(0.2)O_(3.45)N_(0.3) 100.00 90.0986.05 Ex. 22 Li_(3.3)P_(0.5)Ge_(0.5)O_(3.45)N_(0.3) 100.00 89.15 83.72Ex. 23 Li_(3.4)P_(0.4)Ge_(0.6)O_(3.45)N_(0.3) 100.00 88.68 60.47 Ex. 24Li_(3.79)P_(0.01)Ge_(0.99)O_(3.45)N_(0.3) 100.00 84.91 38.14

As is evident from Table 4, for the solid electrolytes of Examples 19 to24 which were represented by a general formulaLi_(a)P_(b)Ge_(c)O_(d)N_(e) where c was 0.01 or greater, the decrease inion conductivity was prevented after storage at the wet atmosphere.Particularly, for those of Examples 20 to 24 where c was 0.1 to 0.99,the decrease in ion conductivity was further prevented. For the solidelectrolyte of Comparative Example 3 where c was 0.005, however, the ionconductivity decreased significantly after the storage.

Table 4 also indicates that high ion conductivity was obtained inExamples 19 to 22 where c was 0.5 or less.

This illustrates that the solid electrolytes of the present inventionproduced by using lithium phosphate and lithium germanate (Li₄GeO₄) as araw material can be represented by a general formulaLi_(a)P_(b)Ge_(c)O_(d)N_(e), and that, when c is 0.01 to 0.99, thedecrease in ion conductivity due to storage in a wet atmosphere isprevented. It has been found that, particularly, c is preferably 0.1 to0.99, and more preferably, 0.1 to 0.5.

Examples 25 to 28 and Comparative Examples 4 to 5

In the second step, a combined method of resistance heating vapordeposition method and ion beam irradiation for introducing nitrogen ionswas used, and the amount of nitrogen introduced was varied so as toobtain solid electrolytes having the compositions shown in Table 5. Inother words, solid electrolytes represented by a general formulaLi_(a)P_(b)Si_(c)O_(d)N_(e), where e was varied in the range of 0.005 to1.0 were produced. The conditions for the resistance heating vapordeposition method were as follows. Lithium orthophosphate and lithiumorthosilicate (Li₄SiO₄) were used as the vapor deposition source. Theion energy of nitrogen ion beam was 100 eV. The current density ofnitrogen ions was controlled so as to obtain solid electrolytes havingthe compositions shown in Table 5. The vapor deposition time was 20minutes. Test cells were produced in the same manner as in Example 1except for the second step.

Subsequently, for each of the test cells, evaluation was madeimmediately after the production and two weeks after the storage in thesame manner as in Example 1. The evaluation results are shown in Table5. Note that the ion conductivity is expressed in relative value withthe ion conductivity obtained immediately after the production of thetest cell set at 100. Further, the ion conductivity immediately afterthe production is expressed in relative value with the ion conductivityof Example 27 set at 100.

TABLE 5 Ion conductivity Im- Ion mediately 2 weeks conductivity afterafter relative to that Solid electrolyte layer production storage of Ex.27 Comp. Li_(3.0)P_(0.8)Si_(0.2)O_(3.8925)N_(0.005) 100.00 83.81 55.26Ex. 4 Ex. 25 Li_(3.0)P_(0.8)Si_(0.2)O_(3.885)N_(0.01) 100.00 87.88 75.00Ex. 26 Li_(3.0)P_(0.8)Si_(0.2)O_(3.75)N_(0.1) 100.00 89.41 89.47 Ex. 27Li_(3.0)P_(0.8)Si_(0.2)O_(3.45)N_(0.3) 100.00 90.00 100.00 Ex. 28Li_(3.0)P_(0.8)Si_(0.2)O_(3.15)N_(0.5) 100.00 88.65 97.37 Comp.Li_(3.0)P_(0.8)Si_(0.2)O_(2.4)N_(1.0) 100.00 85.00 52.63 Ex. 5

As is evident from Table 5, for the solid electrolytes represented by ageneral formula Li_(a)P_(b)Si_(c)O_(d)N_(e), where e was 0.005 to 1.0,the decrease in ion conductivity was prevented after storage at the wetatmosphere irrespective of the value of e.

Further, it has been found that higher ion conductivity was obtained inthe solid electrolytes of Examples 25 to 28 where e was 0.01 to 0.50than those of Comparative Example 4 and 5 where e was 0.005 and 1.0,respectively.

This illustrates that the solid electrolytes of the present inventionproduced by using lithium phosphate and lithium orthosilicate as a rawmaterial can be represented by a general formulaLi_(a)P_(b)Si_(c)O_(d)N_(e), and that, when e is 0.01 to 0.50, high ionconductivity is obtained and the decrease in ion conductivity due tostorage in a wet atmosphere is prevented.

Examples 29 to 31

In the second step, as a lithium oxyacid salt, a lithium oxyacid saltcontaining Si and Ge, a mixture of lithium germanate and lithium borate,or a lithium oxyacid salt containing B and Al was used. The highfrequency power irradiated to the above-listed lithium oxyacid salttarget was controlled so as to obtain solid electrolytes having thecompositions shown in Table 6. Namely, a solid electrolyte composed of anitride of lithium orthophosphate and a lithium oxyacid salt containingSi and Ge (Li₄Si_(0.5)Ge_(0.5)O₄) (Example 29), a solid electrolytecomposed of a nitride of lithium orthophosphate and lithium germanate(Li₄GeO₄) and lithium borate (LiBO₂) (Example 30), and a solidelectrolyte composed of a nitride of lithium orthophosphate and alithium oxyacid salt containing B and Al (LiB_(0.5)Al_(0.5)O₂) (Example31) were produced. Test cells were produced in the same manner as inExample 1 except for the second step.

Subsequently, for each of the test cells, evaluation was madeimmediately after the production and two weeks after the storage in thesame manner as in Example 1. The evaluation results are shown in Table6. Note that the ion conductivity is expressed in relative value withthe ion conductivity obtained immediately after the production of thetest cell set at 100.

TABLE 6 Ion conductivity Im- mediately 2 weeks after after Solidelectrolyte layer production storage Ex. 29Li_(3.0)P_(0.8)Si_(0.1)Ge_(0.1)O_(3.45)N_(0.3) 100.00 90.05 Ex. 30Li_(2.7)P_(0.8)Ge_(0.1)B_(0.1)O_(3.25)N_(0.3) 100.00 84.91 Ex. 31Li_(3.0)P_(0.8)B_(0.1)Al_(0.1)O_(3.45)N_(0.3) 100.00 81.60

As is evident from Table 6, in the solid electrolytes represented by ageneral formula Li_(a)P_(b)M_(c)O_(d)N_(e), even when two differentelements were contained as the element M, the decrease in ionconductivity was prevented without any significant change in ionconductivity after storage in the wet atmosphere.

Examples 32 to 41

In order to evaluate an all solid state batteries using the solidelectrolyte of the present invention, all solid state batteries havingthe structure as shown in FIG. 2 were produced in the followingprocedure.

In the first step, at a predetermined position on a silicon base plate11 having a mirror-finished surface having been oxidized and a surfaceroughness of not greater than 30 nm was placed a metal mask having anopening with a size of 20 mm×12 mm, after which rf magnetron sputteringmethod was performed to form a film composed of platinum. Thereby, afirst current collector 22 having a thickness of 0.5 μm was obtained.

Subsequently, in the second step, on the thus-obtained first currentcollector 22 was placed a metal mask having an opening with a size of 10mm×10 mm, after which rf magnetron sputtering method was performed toform a thin film composed of lithium cobaltate (LiCoO₂). Thereby, afirst electrode (positive electrode layer) 23 having a thickness of 1.0μm was obtained.

Subsequently, in the third step, on the above-obtained first electrode23 was placed a metal mask having an opening with a size of 15 mm×15 mm,after which rf magnetron sputtering method was performed to form a solidelectrolyte 24 having a thickness of 1.0 μm and the composition shown inTable 8.

In the rf magnetron sputtering method, lithium orthophosphate (Li₃PO₄)and the lithium oxyacid salt shown in Table 7 were used as the target.The sputtering gas used here was nitrogen (N₂).

TABLE 7 Lithium oxyacid salt used as target Example 32 Li₄SiO₄ Example33 Li₂SiO₃ Example 34 LiBO₂ Example 35 Li₂GeO₃ Example 36 Li₄GeO₄Example 37 LiAlO₂ Example 38 Li₅AlO₄ Example 39 Li₂CO₃ Example 40 LiGaO₂Example 41 Li₂SO₄

The conditions for the rf magnetron sputtering method were as follows.The chamber internal pressure was 2.7 Pa. The amount of gas introducedwas 10 sccm. The high frequency power irradiated to the lithiumorthophosphate target was 200 W. The sputtering time was two hours.Further, the high frequency power irradiated to the element M-containinglithium oxyacid salt target was controlled so as to obtain solidelectrolytes of lithium oxyacid salt including phosphorus and element Mat a molar ratio of 1:4 and having the compositions shown in Table 8.

In the forth step, on the above-obtained solid electrolyte 24 was placeda metal mask having an opening with a size of 10 mm×10 mm, after whichresistance heating vapor deposition method was performed to form a thinfilm composed of lithium metal. Thereby, a second electrode (negativeelectrode layer) 25 having a thickness of 0.5 μm was obtained.

Further, in the fifth step, on the above-obtained second electrode 25was placed a metal mask having an opening with a size of 20 mm×12 mm,after which rf magnetron sputtering method was performed to form a thinfilm composed of copper such that the thin film completely covered thenegative electrode layer 25 while the thin film was not in contact withthe first current collector 22. Thereby, a second current collector 26having a thickness of 1.0 μm was obtained.

Comparative Example 6

In the third step, with the use of lithium orthophosphate as the target,a thin film composed of lithium phosphorus oxynitride(Li_(2.8)PO_(3.45)N_(0.3)) was formed in the same manner as in Example32. Thereby, a solid electrolyte having a thickness of 1.0 μm wasobtained. A battery was produced in the same manner as in Example 32except for this third step.

[Evaluation]

The all solid state batteries of Examples 32 to 41 and ComparativeExample 6 produced above were stored in a controlled chamber with arelative humidity of 50% and a temperature of 20° C. for two weeks. Foreach of the batteries, alternating current impedance was measuredimmediately after the production and two weeks after the storage. Theconditions for the alternating current impedance measurement were asfollows. The equilibrium voltage was zero, the amplitude of the appliedvoltage was ±10 mV, and the range of the frequency used was 10⁵ to 0.1Hz. Internal impedance was determined from the result of themeasurement.

The results of the internal impedance measurement are shown in Table 8.Note that the internal impedance is expressed in relative value with theinternal impedance obtained immediately after the production of thebatteries set at 100.

TABLE 8 Internal impedance Immediately 2 weeks after after Solidelectrolyte layer production storage Ex. 32Li_(3.0)P_(0.8)Si_(0.2)O_(3.45)N_(0.3) 100.00 111.11 Ex. 33Li_(2.6)P_(0.8)Si_(0.2)O_(3.25)N_(0.3) 100.00 114.86 Ex. 34Li_(2.4)P_(0.8)B_(0.2)O_(3.05)N_(0.3) 100.00 124.74 Ex. 35Li_(2.6)P_(0.8)Ge_(0.2)O_(3.25)N_(0.3) 100.00 133.93 Ex. 36Li_(3.8)P_(0.8)Ge_(0.2)O_(3.45)N_(0.3) 100.00 110.99 Ex. 37Li_(2.4)P_(0.8)Al_(0.2)O_(3.05)N_(0.3) 100.00 118.81 Ex. 38Li_(3.2)P_(0.8)Al_(0.2)O_(3.45)N_(0.3) 100.00 114.20 Ex. 39Li_(2.6)P_(0.8)C_(0.2)O_(3.25)N_(0.3) 100.00 129.46 Ex. 40Li_(2.4)P_(0.8)Ga_(0.2)O_(3.05)N_(0.3) 100.00 121.21 Ex. 41Li_(2.6)P_(0.8)S_(0.2)O_(3.45)N_(0.3) 100.00 131.07 Comp.Li_(2.8)PO_(3.45)N_(0.3) 100.00 700.00 Ex. 6

As shown in Table 8, for the batteries of Examples 32 to 41, anysignificant change in internal impedance was not observed even when theywere stored in the wet atmosphere. For the battery of ComparativeExample 6 without the element M, however, the solid electrolytedeteriorated after the storage, as a result, the internal impedanceincreased significantly.

This indicates that, in Examples 32 to 41, the deterioration of thesolid electrolytes was prevented.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide a solidelectrolyte hardly deteriorates in a wet atmosphere.

1. A solid electrolyte represented by a general formula:Li_(a)P_(b)M_(c)O_(d)N_(e), where M is Si and a, b, c, d and erespectively satisfy a=3.0 to 3.7, b=0.1 to 0.8, c=0.2 to 0.9, d=3.15 to3.75, e=0.1 to 0.5, and b+c=1.0.
 2. An all solid state batterycomprising: a positive electrode; a negative electrode; and the solidelectrolyte in accordance with claim 1 disposed between said positiveelectrode and said negative electrode.
 3. The solid electrolyte inaccordance with claim 1, wherein said formula satisfies b=0.5 to 0.8 andc=0.2 to 0.5.